Author Topic: Conservation of energy/momentum.  (Read 28748 times)

Offline meberbs

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Re: Conservation of energy/momentum.
« Reply #120 on: 10/27/2017 10:53 am »
Just so I do not misunderstand you. You are saying that as I move my arms away from my body, my body moves away from my arms and when I move my arms towards my body , my body moves towards my arms. Is that correct?
That is absolutely correct. It is somewhat counter-intuitive, because it is at variance with our everyday experience.

When you stand on the ground and raise your arms out in front of you, you normally do not detect any movement of your body. That is because friction on the soles of your feet couple you to the planet and so the system that moves back is your body plus the planet. The planet is so much more massive than your arms that the movement is infinitesimal.

If you conducted the experiment on a frictionless surface (an air table or ice rink, say) or in space, the effect would be observable if you took care. Your arms account for about 10% of an average body mass, so if you raise your arms out in front of you, you move their centre of mass forward about a foot (half an arm's length). The centre of mass of the rest of your body will move back about one-ninth of that, i.e. just over one inch. The centre of mass of the combined system (arms plus body) won't move at all.

(Similarly, your body will move down as you raise your arms - again not observed in everyday experience, because of the forces exerted by the surface of the Earth supporting you)


 We can now move to the spring. As I pull the spring away from me and stretch it, the spring will resist and apply a force against the movement of my arms and my body.
So does my body still move backwards (and if so by how much), applying a ever greater force on the spring and vice versa or is its movement hampered by the increasing potential energy of the spring?
Your body will move backwards more. Now the mass of your arms plus the mass of the spring is extended forward, so your body moves backwards more to compensate. For simplicity, assuming the mass of the spring plus the mass of your arms equals 20% of the total mass, using the rest of the same numbers that Nick did means your body would move backwards a bit more than 2 inches.

One thing to keep in mind is that the forces between your hands and one end of the spring are equal and opposite. The forces between the other end of your spring and where it is attached to you are equal and opposite. There is no rule stating that the force on one end of the spring is equal and opposite to the force on the other end of the spring. In fact these have to be unequal at some point to accelerate the center of mass of the spring away from your chest.

Intro physics textbooks discussing systems including springs assume massless springs so that the spring itself does not have to be accelerated which is a very good approximation in many practical cases. If we assume the spring in this case is very light compared to your body, then we just get the same final result as without the spring.

Note that I did not need to mention energy in this description. It is irrelevant at this point and we can move on to discussing energy once it is not going to just add more confusion.

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #121 on: 10/27/2017 03:51 pm »
Just so I do not misunderstand you. You are saying that as I move my arms away from my body, my body moves away from my arms and when I move my arms towards my body , my body moves towards my arms. Is that correct?
That is absolutely correct. It is somewhat counter-intuitive, because it is at variance with our everyday experience.

When you stand on the ground and raise your arms out in front of you, you normally do not detect any movement of your body. That is because friction on the soles of your feet couple you to the planet and so the system that moves back is your body plus the planet. The planet is so much more massive than your arms that the movement is infinitesimal.

If you conducted the experiment on a frictionless surface (an air table or ice rink, say) or in space, the effect would be observable if you took care. Your arms account for about 10% of an average body mass, so if you raise your arms out in front of you, you move their centre of mass forward about a foot (half an arm's length). The centre of mass of the rest of your body will move back about one-ninth of that, i.e. just over one inch. The centre of mass of the combined system (arms plus body) won't move at all.

(Similarly, your body will move down as you raise your arms - again not observed in everyday experience, because of the forces exerted by the surface of the Earth supporting you)


 We can now move to the spring. As I pull the spring away from me and stretch it, the spring will resist and apply a force against the movement of my arms and my body.
So does my body still move backwards (and if so by how much), applying a ever greater force on the spring and vice versa or is its movement hampered by the increasing potential energy of the spring?
Your body will move backwards more. Now the mass of your arms plus the mass of the spring is extended forward, so your body moves backwards more to compensate. For simplicity, assuming the mass of the spring plus the mass of your arms equals 20% of the total mass, using the rest of the same numbers that Nick did means your body would move backwards a bit more than 2 inches.

One thing to keep in mind is that the forces between your hands and one end of the spring are equal and opposite. The forces between the other end of your spring and where it is attached to you are equal and opposite. There is no rule stating that the force on one end of the spring is equal and opposite to the force on the other end of the spring. In fact these have to be unequal at some point to accelerate the center of mass of the spring away from your chest.

Intro physics textbooks discussing systems including springs assume massless springs so that the spring itself does not have to be accelerated which is a very good approximation in many practical cases. If we assume the spring in this case is very light compared to your body, then we just get the same final result as without the spring.

Note that I did not need to mention energy in this description. It is irrelevant at this point and we can move on to discussing energy once it is not going to just add more confusion.

So, I hold my body in position as I extend my arms. If I free my body and bring my arms toward me I can transfer the momentum of my body onto another object without the need for me push against anything?
I am going to push against the object.

Offline meberbs

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Re: Conservation of energy/momentum.
« Reply #122 on: 10/27/2017 04:11 pm »
So, I hold my body in position as I extend my arms.
How do you intend to hold your body in position?

There is no way to truly do so, except by moving something else in the direction your body would have moved.

If I free my body and bring my arms toward me I can transfer the momentum of my body onto another object without the need for me push against anything?
It is unclear what you are trying to say here. Starting from a position with your arms extended and you at rest, pulling your arms towards you will result in your body moving forward slightly ending up stationary with your arms near to your body as we previously discussed. Nothing else happens unless you hit something while making that motion.

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #123 on: 10/28/2017 09:09 am »
So, I hold my body in position as I extend my arms.
How do you intend to hold your body in position?

There is no way to truly do so, except by moving something else in the direction your body would have moved.

If I free my body and bring my arms toward me I can transfer the momentum of my body onto another object without the need for me push against anything?
It is unclear what you are trying to say here. Starting from a position with your arms extended and you at rest, pulling your arms towards you will result in your body moving forward slightly ending up stationary with your arms near to your body as we previously discussed. Nothing else happens unless you hit something while making that motion.

Substitute my body for the body of the device (stator). Substitute my arms for the field flux lines and read the description again.
 It would be a good idea to add another magnet onto the belt in the collision scenario of the two counter rotating motors. You are correct read belt instead of body.

Offline Nick

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Re: Conservation of energy/momentum.
« Reply #124 on: 10/28/2017 01:41 pm »
So, I hold my body in position as I extend my arms. If I free my body and bring my arms toward me I can transfer the momentum of my body onto another object without the need for me push against anything?
Yes, of course you can. If you hit another object while your body is in motion (returning to its original position) that second object will, in general, move off in some direction, and your body will not return to rest in its original position. You will have  moved the second object but your body will also have moved from its original position and state of rest.

What you have described is effectively a complicated way of throwing the second object. Instead of simply holding it and throwing it away, you will have to position the second object so that part of you can collide with it while you move your arms.

All of these situations obey Newton's second & third laws. The second, F=ma, requires an external force in order to accelerate a body. The third, that every action has an equal and opposite reaction, means that when you add up the forces between all the components of a complex body (arms, legs, torso, stators, springs, magnets, nearby tennis balls, whatever) there is no net force on that total complex body as a whole. And hence no acceleration. It doesn't matter whether the forces between the individual components are provided by muscular contractions, springs, mechanical devices, chemical explosions, or by magnetic, electric or gravitational fields. The net total of all those forces is zero, and hence there can be no acceleration of the set of components in total. That doesn't mean than you can't move one component away from the others - but the centre of mass of the ejected component and those left behind doesn't move.

Exactly the same is true of a rocket. If a rocket is fired in empty space, the vehicle moves off in some direction, and the exhaust in the opposite direction. What  we don't usually think about is that, as the exhaust hurtles away from the vehicle, the centre of mass of the vehicle plus exhaust, taken together, doesn't move at all. As the 100+ tons of the Apollo/SIVB combination set off for the Moon, a similar quantity of propellant was sent in the opposite direction to whatever fate awaited it in later interactions with Earth & Sun.

Offline meberbs

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Re: Conservation of energy/momentum.
« Reply #125 on: 10/28/2017 03:40 pm »
So, I hold my body in position as I extend my arms.
How do you intend to hold your body in position?

There is no way to truly do so, except by moving something else in the direction your body would have moved.

If I free my body and bring my arms toward me I can transfer the momentum of my body onto another object without the need for me push against anything?
It is unclear what you are trying to say here. Starting from a position with your arms extended and you at rest, pulling your arms towards you will result in your body moving forward slightly ending up stationary with your arms near to your body as we previously discussed. Nothing else happens unless you hit something while making that motion.

Substitute my body for the body of the device (stator). Substitute my arms for the field flux lines and read the description again.
 It would be a good idea to add another magnet onto the belt in the collision scenario of the two counter rotating motors. You are correct read belt instead of body.
I cannot make those substitutions, because they don't make sense.  At a basic level a magnetic field does not have mass, while your arms do. Trying to make an analogy there is flawed. There also isn't an equivalent to extending your arms when it comes to magnetic fields.

Also, you seem to be switching from a linear motion system to a rotating one.

You also have to explain what you mean by "hold the body still"

You talk a bout a "device" and "collisions" The device you have shown a video of does not even come close to matching your description here (to start with it only has one motor) and the device you previously attempted to describe involving belts and magnets did not seem to be set up in such a way that a collision could occur. I'd ask for a better description, but most of your descriptions seem to assume that the person on the other end is psychic, so I will have to ask for a detailed labelled diagram.

Before moving on to this new system, it would be helpful if you acknowledged the previous description that when stretching a spring attached to you by extending your arms, the spring does not change the result other than increasing the mass of what you extend forward to be mass of arms plus string rather than mass of arms.

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #126 on: 10/29/2017 08:50 am »
So, I hold my body in position as I extend my arms.
How do you intend to hold your body in position?

There is no way to truly do so, except by moving something else in the direction your body would have moved.

If I free my body and bring my arms toward me I can transfer the momentum of my body onto another object without the need for me push against anything?
It is unclear what you are trying to say here. Starting from a position with your arms extended and you at rest, pulling your arms towards you will result in your body moving forward slightly ending up stationary with your arms near to your body as we previously discussed. Nothing else happens unless you hit something while making that motion.

Substitute my body for the body of the device (stator). Substitute my arms for the field flux lines and read the description again.
 It would be a good idea to add another magnet onto the belt in the collision scenario of the two counter rotating motors. You are correct read belt instead of body.
I cannot make those substitutions, because they don't make sense.  At a basic level a magnetic field does not have mass, while your arms do. Trying to make an analogy there is flawed. There also isn't an equivalent to extending your arms when it comes to magnetic fields.

Also, you seem to be switching from a linear motion system to a rotating one.

You also have to explain what you mean by "hold the body still"

You talk a bout a "device" and "collisions" The device you have shown a video of does not even come close to matching your description here (to start with it only has one motor) and the device you previously attempted to describe involving belts and magnets did not seem to be set up in such a way that a collision could occur. I'd ask for a better description, but most of your descriptions seem to assume that the person on the other end is psychic, so I will have to ask for a detailed labelled diagram.

Before moving on to this new system, it would be helpful if you acknowledged the previous description that when stretching a spring attached to you by extending your arms, the spring does not change the result other than increasing the mass of what you extend forward to be mass of arms plus string rather than mass of arms.

You have completely missed the plot here. The spring discussion is neither here or there. To expend so much energy to stop, start, stop, start, a millimetre at a time is, in my opinion, about as useful as a chocolate teapot. The point is to use  the scenario and the comments made (thank you to everyone who commented) as a platform to hopefully make some meaningful progress.
If it is your desire for me to be in error may I point to my comments on substitutes. The spring should be substituted for the field flux lines ( potential energy) and my arms for the armature reaction for the device in the video.

On a completely different subject, using electromagnets in the two counter rotating motor, belt scenario,  both the collision and push back  instances look cautiously promising.

Offline meberbs

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Re: Conservation of energy/momentum.
« Reply #127 on: 10/29/2017 03:31 pm »
You have completely missed the plot here. The spring discussion is neither here or there. To expend so much energy to stop, start, stop, start, a millimetre at a time is, in my opinion, about as useful as a chocolate teapot. The point is to use  the scenario and the comments made (thank you to everyone who commented) as a platform to hopefully make some meaningful progress.
Based on this comment, it sounds like you still think that you can have net motion in the scenario with the spring. If you still think this, you still do not understand that scenario, and there is no point in discussing more complicated scenarios with you.

If it is your desire for me to be in error
It is my desire for you to stop being in error.

On a completely different subject, using electromagnets in the two counter rotating motor, belt scenario,  both the collision and push back  instances look cautiously promising.
On that subject, I just stated in my precious post that you have failed to describe that situation in such a way where a collision could occur. If you want to discuss it, you are going to have to find a better way to describe what in the world you are talking about. (But first lets settle the matter of you understanding that you can move exactly nowhere in the spring scenario no matter how much effort you exert.)

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #128 on: 10/30/2017 11:50 am »
Basic sketch attached, there are numerous variations that can be made ie change position of hull magnets, add more magnets to belts, add a braking zone per cycle etc, etc, etc.

Offline meberbs

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Re: Conservation of energy/momentum.
« Reply #129 on: 10/30/2017 03:10 pm »
Basic sketch attached, there are numerous variations that can be made ie change position of hull magnets, add more magnets to belts, add a braking zone per cycle etc, etc, etc.
Thank you, it is much more clear what you are talking about now.

I still have no idea what you mean by a "collision" though because that setup does not include anything that would collide with anything else.

Before we discuss this situation you still haven't answered an important question: Do you understand that the scenarios described with stretching a spring that is attached to you you cannot move anywhere at all no matter how much time or effort you put in?

Assuming the answer is yes, so that there is a point in moving on to the device with the belts, then you'll understand a request to add a block of wood with the same mass as the magnet to each belt on the farthest point of each belt from the magnet. This will keep everything balanced, so that we don't have to discuss the slight useless back and forth of the hull that would otherwise be needed to balance momentum when both magnets on the belts are moving in the same direction.

Besides that, your setup does absolutely nothing interesting whatsoever and will not cause the hull of the ship to budge a single inch, doesn't even matter what orientation the magnets are in. An explanation of why you think otherwise would help in figuring out what exactly it is that you still don't understand.

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #130 on: 10/30/2017 04:03 pm »
The belt magnets and the hull magnets are repelling each other so we will have the facing poles as north. I will go through some scenarios soon, mostly with electromagnets and hopefully someone will have some constructive comments. 

Offline meberbs

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Re: Conservation of energy/momentum.
« Reply #131 on: 10/30/2017 06:31 pm »
Last chance, if you have any interest in having a productive conversation, you need to answer this question. If you continue to ignore it and insist on moving on to another situation with more moving parts without demonstrating that you actually understand the conclusion from the previous discussion, no one is going to have any interest in continuing to converse with you.

Before we discuss this situation you still haven't answered an important question: Do you understand that in the scenarios described with stretching a spring that is attached to you, you cannot move anywhere at all no matter how much time or effort you put in?

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #132 on: 10/31/2017 11:00 am »
The device is enclosed in the box (see diagram) and as I stated earlier, we will add another magnet of the same mass to each belt (all the magnets are electromagnets which at the start are all switched off). The additional magnets will be positioned so that when the motors are switched on, the box will not move. So we switch the motors on and the belts begin to move. When two of the belt magnets are approaching the hull magnets we can switch off the motors and switch on the belt and hull electromagnets. The belt magnets are now able to free wheel towards the hull magnets and since the magnetic orientation is repelling, they collide with the repelling magnetic fields imparting their momentum onto the hull magnets causing the body to move. We do not have to concern ourselves with the slight bounce back. Other configurations ie pull and push back can be considered.

Online Basto

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Re: Conservation of energy/momentum.
« Reply #133 on: 10/31/2017 02:53 pm »
Your belt magnets are part of the same system as the hull magnets. No motion would occur as the belt magnets would push back with an equal amount of force as the hull magnets.

Itís the same reason you cannot lift a manhole cover you are standing on.

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #134 on: 10/31/2017 03:05 pm »
Your belt magnets are part of the same system as the hull magnets. No motion would occur as the belt magnets would push back with an equal amount of force as the hull magnets.

Itís the same reason you cannot lift a manhole cover you are standing on.


Remember, the belt magnets are moving and the hull magnets are stationary before the collision occurs.

Offline whitelancer64

Re: Conservation of energy/momentum.
« Reply #135 on: 10/31/2017 03:39 pm »
Your belt magnets are part of the same system as the hull magnets. No motion would occur as the belt magnets would push back with an equal amount of force as the hull magnets.

Itís the same reason you cannot lift a manhole cover you are standing on.


Remember, the belt magnets are moving and the hull magnets are stationary before the collision occurs.

It doesn't matter that you've made a set of magnets move and have another set fixed to the wall, it's still just the system pushing against itself. If you measure all the forces in the system they will turn out to be balanced. It's the same error you made with the spring, just with magnets this time. Moving the spring simply shifted the mass in the system around without it actually moving anywhere. The same thing happens with the magnets.

If you're not convinced, then maybe you should actually build this electromagnet contraption you've sketched out. Put it in a small cart with well-lubricated wheels, set it on a level, smooth floor, turn it on, see if it moves. Watch as it just rocks back and forth a bit without going anywhere.
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Offline Bob012345

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Re: Conservation of energy/momentum.
« Reply #136 on: 10/31/2017 03:41 pm »
Your belt magnets are part of the same system as the hull magnets. No motion would occur as the belt magnets would push back with an equal amount of force as the hull magnets.

Itís the same reason you cannot lift a manhole cover you are standing on.

We'll, actually you can in principle but probably impossible in practice..... same as skateboarders lifting the board they ride on. But only because both can react against the earth but the principle behind what I think you meant is correct.  :)

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #137 on: 10/31/2017 04:10 pm »
Your belt magnets are part of the same system as the hull magnets. No motion would occur as the belt magnets would push back with an equal amount of force as the hull magnets.

Itís the same reason you cannot lift a manhole cover you are standing on.

We'll, actually you can in principle but probably impossible in practice..... same as skateboarders lifting the board they ride on. But only because both can react against the earth but the principle behind what I think you meant is correct.  :)

When you turn the motors off, nature is in check. It only has one square to move and that is to transfer the momentum to the body.

Offline Bob012345

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Re: Conservation of energy/momentum.
« Reply #138 on: 10/31/2017 04:25 pm »
Your belt magnets are part of the same system as the hull magnets. No motion would occur as the belt magnets would push back with an equal amount of force as the hull magnets.

Itís the same reason you cannot lift a manhole cover you are standing on.

We'll, actually you can in principle but probably impossible in practice..... same as skateboarders lifting the board they ride on. But only because both can react against the earth but the principle behind what I think you meant is correct.  :)

When you turn the motors off, nature is in check. It only has one square to move and that is to transfer the momentum to the body.

My comment only concerned the manhole illustration. Your system has nothing to react against.
« Last Edit: 10/31/2017 04:27 pm by Bob012345 »

Offline hop

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Re: Conservation of energy/momentum.
« Reply #139 on: 10/31/2017 04:35 pm »
The belt magnets and the hull magnets are repelling each other so we will have the facing poles as north. I will go through some scenarios soon, mostly with electromagnets and hopefully someone will have some constructive comments. 
Rather than coming up with more complicated contraptions, you would be far better served by taking the time to understand why your previous ideas were wrong. Once you do, you should realize why adding more springs/magnets/belts etc cannot change the basic outcomes, which will save you a lot of time and effort in the long run.

As others have suggested, a introductory physics course might help. You might also benefit from spending some time on https://www.lhup.edu/~dsimanek/museum/unwork.htm (spoiler: none of these devices do anything interesting)

Nothing you have posted here is "New Physics" (the subject of the forum) or has any novel or interesting implications concerning "Conservation of energy/momentum"

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